WIP:Solver

The following topic will be treated in this branch:

1. generalization space operator interface for arbitrary differentiable operators
2. get a common nameing scheme for solver/InverseOperators

A possible nameing scheme is:

• Methods which are calling op.systemMatrix() will be named *Solver, eg OEMSolver, ISTLSolver
• Methods that uses only the Fem::Operator will be named *InverseOperator, eg. CGInvereOperator

@tobias.malkmus and @stefan.girke take care of it.